This invention relates to an in-line type electron gun structure, more particularly improvement of the electrode construction thereof.
Generally, in order that three electron guns usually used in a color picture tube have an excellent convergence characteristic, an electron gun structure has been used in which three electron guns are closely assembled in an integral form.
FIG. 1 of the accompanying drawing illustrates one example of a prior art in-line type electron gun structure in which a cathode holder 1 is provided to hold three parallelly disposed cathode electrodes 2 which are arranged in line. The cathode electrodes 2 contain heaters 3 to constitute electron beam emitting members. Above the cathode electrodes 2 are disposed in succession a first grid assembly 4 which controls the electron beams, a second grid assembly 5 for accelerating the electron beams and third and fourth grid assemblies 6 and 7 constituting an electron lens, various grid assemblies being supported by glass beads 8. By the action of these grid assemblies, the electron beams are caused to impinge upon selected phosphor picture elements of the color picture tube.
The third and fourth grid assemblies 6 and 7 are also called main lens electrodes and respectively comprise three sleeve electrodes 6a, 6b and 6c and 7a, 7b and 7c which are in register with the respective electron beam emitting members. A cylindrical auxiliary electrode 9 is coaxially provided for each sleeve electrode. The purpose of the auxiliary electrodes 9 is to prevent the side walls of the third and fourth grid assemblies from affecting the electric field created in the third and fourth grid assemblies 6 and 7. Accordingly, it is not necessary to provide the auxiliary electrodes 9 as long as the sleeve electrodes, 6a, 6b, 6c, 7a, 7b, and 7c have a length that can satisfy a predetermined condition and the accuracy of circular inner shape of these sleeve electrodes is sufficiently high.
FIG. 2 is a graph showing the relationship between the ratio of the sleeve length L to the sleeve diameter D and the focusing characteristic of an electron beam. The graph was obtained by using main lens electrodes which were produced by machining non-magnetic stainless plates which otherwise have been prepared by press work. In FIG. 2, the focusing characteristic is expressed by a ratio of the longitudinal dimension B of the beam spot to the lateral dimension A of the beam spot at the central portion of the fluorescent screen of the color picture tube. As can be noted from FIG. 2, it is ideal that the longitudinal to lateral ratio B/A of the beam spot is 1.0, that is, the cross-section is a circle. However, it has been found that error of .+-.5%, that is, the ratio of from 0.95 to 1.05 does not affect the focusing characteristic of the color picture tube. FIG. 2 also shows that in order to obtain a satisfactory focusing characteristic for a color picture tube and to eliminate auxiliary electrodes 9, the ratio L/D should be higher than about 50%.
FIG. 3 is a graph showing the relationship between the degree of a perfect circle of the sleeves and the focusing characteristic of a color picture tube. Thus, the abscissa shows the degree of a perfect circle in microns (as represented by B-A) while the ordinate shows the ratio of a diameter of a beam spot core portion which produces a uniform, high brightness to a diameter of a beam spot halo portion where the picture image is blurred, when the beam spot impinges upon the peripheral portion of the picture screen. As can be seen from FIG. 3, when the degree of a perfect circle of the inner periphery of the sleeve decreases below about 40 microns, the size of the halo portion increases rapidly to impair the quality of the color picture tube.
Summarizing the above description, in order to elimate the auxiliary electrodes of the main lens electrodes, the L/D ratio should be higher than about 50% and the degree of a perfect circle of the inner periphery of the sleeves shoud be less than about 40 microns.
Two examples of the prior art method of manufacturing the main lens electrode will now be described. As shown in FIGS. 4 and 5, the main lens electrode comprises an oblong main body 10 made of non-magnetic stainless steel and formed with sleeve electrodes 7a, 7b and 7c having an inner diameter of Dmm and acting as a lens. The periphery of each sleeve electrode extends inwardly from the top 11 of the main body. To form sleeve elctrodes 7a, 7b and 7c, the main body 10 is formed as a sleeve with a press. Then, circular openings 12 having a diameter d smaller than that of the inner diameter D of the sleeve electrodes are formed through the top plate 11 of the main body, as shown in FIG. 6. Then, as shown in FIG. 7, the top plate 11 is mounted on a support 13 with the circular opening 12 aligned with openings 30, and punches 14 are inserted into the openings 12 to inwardly bend the portions of the top plate 11 about the openings 12, thus forming the sleeve electrodes 7a, 7b and 7c having a predetermined inner diameter D.
According to this prior art method, however, the upper limit of the ratio L/D is at most 24% which is less than the ratio at which the auxiliary electrodes 9 can be eliminated.
According to another prior method of improving the ratio L/D shown in FIGS. 8, 9 and 10, recesses 15 having an inner diameter slightly smaller than the inner diameter D of the sleeve electrodes are formed in the top plate 11 of the main body. Then, as shown in FIG. 9, circular openings 16 are formed through the bottom walls of the recesses 15. Finally, as shown in FIG. 10, the peripheries of the openings 16 are bent downwardly to form sleeve electrodes 17 having a predetermined inner diameter D.
Although this method can improve the ratio L/D over the method shown in FIGS. 6 and 7, the ratio is at most about 40% which is lower than the condition necessary to eliminate the auxiliary electrodes. According to the latter method, when the sleeve is formed as shown in FIG. 10, circumferential strain will be created at the lower opening 18 of the sleeve so that the degree of a perfect circle near the upper end of the sleeve amounts to 15 to 25 microns, whereas that of the lower opening 18 greatly increases to 40 to 70 microns. For this reason, assuming a practical ratio L/D=40%, it is necessary to heat the electrode after press forming and then to correct the degree of the perfect circle.